In television signal processing it is often required that a horizontal time base signal be provided to various receiver functions in the absence of a horizontal drive signal. Horizontal drive is defined as a horizontal time base signal that is continuous during the vertical interval, unlike horizontal sync which-is blanked during the vertical interval. When a video signal is received, it is necessary to generate a horizontal drive signal which is synchronized to the received horizontal sync. When a signal is not received it is necessary to provide a horizontal time base signal that is fairly close in frequency and pulse width to horizontal drive. In the past this function of generating the horizontal time base signal has been done using either a phase locked loop or a synchronized oscillator. Each of these approaches can satisfy the objectives of providing a synchronized horizontal drive signal when a video input is received and a free-running horizontal time base signal otherwise; however, each has significant negative attributes. The phase locked loop suffers due to its cost and complexity as well as its potential phase jitter. Synchronized oscillators, usually either blocking oscillators or synchronized astable multivibrators, both suffer from the problem that when they are synchronized to horizontal sync, the output horizontal time base signal pulse width is determined by the oscillator components rather than the incoming horizontal sync. Due to this characteristic, events relying on the horizontal drive pulse width may vary as a function of temperature and component aging. A further problem with both the phase locked loop and synchronized oscillator approaches is that the output horizontal time base signal does not contain the equalizing pulses of the received horizontal sync signal.
The present invention provides a circuit and a method for providing a horizontal time base signal that approximates horizontal drive when no video signal is present and when horizontal sync is absent during the vertical interval of a received video signal. This invention also provides a pass-through of the incoming horizontal sync during the times when said horizontal sync is present, thus eliminating timing and pulse width uncertainties.